So-called no-rinse processes are now generally known in connection with the chemical treatment of metal surfaces, for example, for the subsequent application of lacquers, adhesives and/or plastics. In no-rinse processes, the metal surface is cleaned in a first stage to remove oil, dirt and other residues. Any residues of chemicals from this first stage are removed by rinsing with water. In the following stage of the process, the clean metal surface is wetted with an aqueous bath solution which is not rinsed off, but instead is dried in situ on the metal surface and converted there into a solid film of the bath constituents. Surface quality, particularly in regard to corrosion prevention and the adhesion of subsequently applied covering layers, can be substantially improved by coatings such as these.
Originally, treatment solutions containing hexavalent chromium compounds were frequently proposed in the extensive prior-art literature on this subject. Due to the toxic nature of those compounds, the processes in question or rather the rinsing waters accumulating in them require elaborate effluent treatment.
The use of treatment solutions containing hexavalent and trivalent chromium salts together with film-forming agents are described, for example, in German Applications No. 17 69 582 and No. 29 03 311. In the first of these two references, an alkali silicate, for example, is said to be used as the inorganic film-forming agent. According to the second reference, polyacrylic acid is used as the organic film-forming agent. Due to the presence of hexavalent chromium in the aqueous bath liquids, these materials are unsuitable for use in the food industry.
German Application No. 27 11 431 describes a process for the surface treatment of metals, particularly iron, zinc, and aluminum, in which the cleaned metal surface is said to be wetted with an aqueous acid solution containing chromium(III) ions, phosphate ions and finely particulate silica. This treatment solution may additionally contain acetate ions, maleate ions, zinc ions and/or manganese ions. Although there is no need in this case to use the toxic chromium-(VI) ion, acidic dispersions containing silica and phosphate ions have the disadvantage of a limited pot life due to flocculation.